The present invention relates to disc drives. More specifically, the present invention relates to a spindle motor in a disc drive and a method of making the motor.
A typical disc drive includes one or more magnetic discs mounted for rotation on a hub or spindle. A typical disc drive also includes one or more transducers supported by a hydrodynamic air bearing which flies above each magnetic disc. The transducers and the hydrodynamic air bearing are collectively referred to as a data head. A drive controller is conventionally used for controlling the disc drive system based on commands received from a host system. The drive controller controls the disc drive to retrieve information from the magnetic discs and to store information on the magnetic discs.
An electromechanical actuator operates within a negative feedback, closed-loop servo system. The actuator moves the data head radially over the disc surface for track seek operations and holds the transducer directly over a track on the disc surface for track following operations.
Information is typically stored on the magnetic discs by providing a write signal to the data head to encode flux reversals on the surface of the magnetic disc representing the data to be stored. In retrieving data from the disc, the drive controller controls the electromechanical actuator so that the data head flies above the magnetic disc, sensing the flux reversals on the magnetic disc, and generating a read signal based on those flux reversals. The read signal is then decoded by the drive controller to recover the data represented by flux reversals stored on a magnetic disc, and consequently represented in the read signal provided by the data head.
Accurate positioning of the data head over a track on the disc is of great importance in writing data to the disc and reading data from the disc.
In current generation disc drive products, the most commonly used type of actuator is a rotary moving coil actuator. The discs themselves are typically mounted in a "stack" on the hub structure of a brushless DC spindle motor. The rotational speed of the spindle motor is precisely controlled by motor drive circuitry which controls both the timing and the power of commutation signals directed to the stator windings of the motor. The hub structure is rotatably coupled to a shaft via a set of bearings. The hub includes a sleeve or back iron portion which carries a magnet. Interaction between the magnet and the stator causes controlled rotation of the hub about the shaft.
Motor acoustic levels are related to the degree of alignment between the stator and the magnet, both axially (along the axis of rotation of the hub) and radially. Alignment of the magnet and stator in both the axial and radial directions is difficult to control during volume production of disc drive motors due to tolerances incurred in producing the magnet and stator parts.
In addition, alignment in the axial direction is difficult because the "magnetic center" of the hub does not necessarily correspond to the "geometric center". In other words, a certain imbalance in the flux field can be caused by the configuration of the back iron. This results in the magnetic center of the magnet relative to the stator being slightly different than the precise geometric center of the two parts.
Currently, the magnet and stator are axially aligned using nominal dimensions. However, the tolerances on the parts allow up to 0.3 mm of misalignment between the two parts. Further, even if the two parts are perfectly geometrically aligned, they may well not be magnetically aligned. Both of these misalignments can be problematic during operation of the drive motor.